As for the pipe; don't try to fix it. You may be able to clear it, but
that would only be a temp move it would only do the same thing again.
Replace it with a water tight plastic pipe. They cause less problems since
the old iron ones tended to leak just enough to attract tree roots.

YOu could run polyflex, or, if it's a straight shot to wherever it goes,
even the next size smaller PVC down the inside of the cast iron,
after rooting it out. That would save digging a new trench.